Why is he peeing in the bathroom?!

My 10 year old male, neutered moggy has suddenly started peeing in the bathroom. Second offence as far as I know, he was caught in the act by DS this morning, time before that was a few weeks previous. Both times he's done it when DH has left the mat you stand on when getting out the shower on the floor rather than hanging it over the towel rail. He peed on the edge of the mat and the side of the bath tub. He's a very happy, affectionate cat and in good health. He has the occasional half hearted scrap with his brother but they generally rub along well together. I don't understand why he's started doing this as neither of mine have ever done anything like it before. Well, actually there were a couple of incidents years ago of one of them peeing in the actual bath but we never determined the culprit.
Anyone any ideas to why he might have started doing it and how I can stop it from happening again?
We've got a cat flap so they can come and go as they please.